Retaining walls are commonly used for architectural and site development applications. The wall facing must withstand very high pressures exerted by backfill soils. Reinforcement and stabilization of the soil backfill is commonly provided by grid-like sheet materials that are placed in layers in the soil fill behind the wall face to interlock with the wall fill soil and create a stable reinforced soil mass. Connection of the reinforcing material to the elements forming the wall holds the wall elements in place and minimizes soil backfill pressures.
A preferred form of grid-like tie-back sheet material used to reinforce the soil behind a retaining wall structure, known as an integral geogrid, is commercially available from The Tensar Corporation of Atlanta, Ga. ("Tensar") and is made by the process disclosed in U.S. Pat. No. 4,374,798 ("the '798 patent"), the subject matter of which is incorporated herein in its entirety by reference. However, other forms of grid-like tie-back sheet materials have also been used as reinforcing means in the construction of retaining walls, and the instant inventive concepts are equally applicable with the use of such materials. In any event, difficulties are encountered in providing a secure interconnection between the reinforcing means and the wall elements, especially in areas of high seismic activity.
In a brochure entitled "Concrete Geowall Package", published by Tensar in 1986, various retaining wall structures are shown using full height cast concrete panels. In one such retaining wall structure short strips, or tabs, of geogrid material, such as shown in the '798 patent, are embedded in the cast wall panels. On site, longer strips of geogrid are used to reinforce the wall fill, creating a stable soil mass. To connect the geogrid tabs to the reinforcing geogrid, the strands of one portion of geogrid are bent to form loops, the loops are inserted between the strands of the other portion of geogrid so that the loops project out of the second portion of geogrid, and a rod is passed through the loops on the opposite side of the second portion to prevent the loops being pulled back through, thereby forming a tight interconnection between the two portions of geogrid, sometimes referred to as a "Bodkin" joint.
Use of full height pre-cast concrete wall panels for wall-facing elements in a retaining wall requires, during construction, that the panels be placed using a crane because they are very large, perhaps 2 by 12 feet or even larger and, as a result, are quite heavy such that they cannot be readily man-handled. To avoid such problems in the use of pre-cast wall panels other types of retaining wall structures have been developed. For example, retaining walls have been formed from modular wall blocks which are typically relatively small cementitious blocks as compared to cast wall panels. The assembly of modular wall blocks usually does not require heavy equipment. Such modular wall blocks can be handled by a single person and are used to form retaining wall structures by arranging a plurality of blocks in courses superimposed on each other, much like laying of brick or the like. Each block includes a body with a front face which forms the exterior surface of the formed retaining wall.
Modular wall blocks are formed of concrete, commonly mixed in a hatching plant with only enough water to hydrate the cement and hold the unit together. Such blocks are commercially made by a high-speed process which provides a mold box having only sides, without a top or bottom, positioned on top of a steel pallet which contacts the mold box to create a temporary bottom plate. A concrete distributor box brings concrete from the batcher and places the concrete in the mold box and includes a blade which levels the concrete across the open top of the mold box. A stripper/compactor is lowered into the open, upper end of the box and contacts the concrete to imprint the block with a desired pattern and compresses the concrete under high pressure. The steel pallet located at the bottom of the mold box resists this pressure.
A vibrator then vibrates the mold box to aid in concrete consolidation. After approximately two to four seconds, the steel pallet is moved away from the bottom of the mold box which has been positioned above a conveyor belt. The stripper/compactor continues to push on the formed concrete to push the formed modular wall block out of the mold box onto the conveyor belt. This process takes about seven to nine seconds to manufacture a single wall block. The formed wall block is cured for approximately one day to form the final product.
With this high-speed method of construction, it is not practical to embed short strips or tabs of grid-like material or the like in the blocks with portions extending therefrom in the manner of the pre-cast wall panels shown in the Tensar brochure, in order to enable interconnection with a grid-like reinforcing sheet material such as by a Bodkin-type connection or the like. Therefore, other means for securing the reinforcing grid to selected modular blocks used to construct a retaining wall have had to be devised. Most such techniques actually secure end portions of a sheet of reinforcing grid between layers of wall blocks, relying primarily on the weight of superimposed blocks to provide a frictional engagement of the reinforcing means with the retaining wall. The nature of the very rough surface of such cementitious modular wall blocks tends to abrade, and thereby weaken, a polymeric sheet reinforcing material at the very point of interconnection with the retaining wall. Moreover, and most importantly, reliance on the weight of superimposed blocks to provide the primary grid-to-block connection strength is ineffective during an earthquake or other such seismic event where vertical accelerations, i.e., the actually momentary lifting of upper courses of wall blocks, decrease or totally eliminate the weight of superimposed blocks, thereby significantly reducing or destroying the connection strength and jeopardizing the stability of the retaining wall and the soil mass retained thereby.